Will you stay in touch? Will Facebook messages, emails and postcards suffice to keep the bond strong?
They made you happy just by their presence in your life. Their physical absence will not diminish the joy you felt in their company, and thanks to modern technology you can tell them that as often as you want.
Tell them about funny little things that happened in your day. Tell them what made you think of them in that moment. Tell them you miss them and look forward to seeing them again.
Ask them how they're settling in. Ask them what they get up to in their free time. Ask them about the weather. Ask them what they missed about their home country that they're happy to have again, and what they didn't miss and want to vent about.
Send them letters, postcards, or care packages. Send them pictures of what you're up to and stickers on messages. Send them your love.
The world isn't as big as it used to be, but only you can make it feel smaller. Your friendship is its own country - be good to its citizens. Say hello instead of goodbye.